 Abstract | Regional groundwater discharge zones accumulate large quantities of salt as hypersaline subsurface brines in arid and semi-arid environments. However, relatively little is known about the links between hydraulic and hydrochemical processes affecting water and solutes below the large complexes of playa lakes that are common throughout Australia and many other parts of the world. This thesis has investigated, using a combination of hydraulic, hydrochemical and isotopic methods, variability in hydrochemical and hydraulic processes at a range of scales (10 -2 - 10 3 m) in the Raak Plain groundwater discharge complex of south-eastern Australia.
The study found that subsurface brine compositions below the playas at Raak Plain are essentially evapo-concentrated versions of the seawater-like inflow, despite variations in physical playa characteristics such as size, shape, and the presence / absence of surface water bodies and salt crusts. Small differences in ionic ratios in the top 5 cm of sediment pore waters occur due to seasonal precipitation and dissolution of surface salt efflorescences, and fractionation due to different rates of diffusive transport of ions across steep concentration gradients. Chemical and isotopic data and hydraulic modelling suggest that variability in playa physical characteristics is due to different vertical hydraulic gradients across a 30 m thick aquitard separating the regional aquifer from the local dune recharge areas. This results in locally recharged groundwater either flowing directly onto the playa surface, forming surface water bodies and salt crusts, or circulating through the deeper aquifer system before being discharged. The hydraulic modelling, in particular, highlighted the delicate balance between local and regional flow systems in groundwater discharge complexes.
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